The purpose of this research study is to identify factors and genes (the nucleic acid material that determines the makeup of the human body) that may be associated with how children respond to pain medication, like morphine, which are called opioids. Though these pain medicines are used every day, some children have bad reactions from their use, like breathing problems, sedation, etc. It is not possible to completely predict which pain medicine is 'right' for a child. The investigators want to study factors that may be associated with pain sensitivity, pain medicine requirements after surgery, and their side-effects. The investigators expect that the information obtained in this research study will help to develop effective, safer, and tailored treatment options in the future.